Some known metals may repeat exothermic absorption and endothermic desorption (or dissociation) of hydrogen, through hydrogenating the metal and dehydrogenating the metal hydride formed, respectively.
In what follows those metals are referred to as metal hydrides even after they have dissociated the hydrogen and restored metal states.
Systems utilizing metal hydrides for obtaining cold heat and hot heat have been disclosed in, for example, Japanese Patent Publication Sho 68-4111.
However, equilibrium hydrogen pressure during refrigeration becomes too low for such prior art system to serve as a cold heat source at -50.degree. C., since hydrogenation of metals then proceeds very slowly. Also, it is difficult with the prior art systems to efficiently extract cold heat in a controlled manner at an arbitrary temperature in the range of -20.degree. C. and -50.degree. C., say. Furthermore, prior art systems fail to provide desirable cold heat when the temperature of the heat radiator becomes high, since equilibrium hydrogen pressure becomes very high at high temperatures.